This invention relates to diode rectifiers in general, and particularly to a diode rectifier assembly in which a plurality of diodes are connected in parallel.
In order to provide high current handling capacity from semi-conductor diodes, it is common to connect a number of individual diodes in parallel. Such diodes are often mounted in parallel within a hermetically sealed metal container having two leads. One lead represents the anode connection of the diodes, and the other lead represents the cathode connection of the diodes. When the diodes are so connected, they must be very carefully matched in characteristics in order to avoid imbalances in the amount of current conducted by the individual diodes. Such imbalances of current flow can result in heating of a diode which carries greater part of the current, which in turn can result in thermal runaway. Thermal runaway is a condition in which as the diode temperature increases its conductance increases thereby causing the diode to pass an even greater part of the current of the rectifier. Once a thermal runaway condition has been reached, increasing temperature and current flow in the individual diode will eventually destroy that diode. Quite often the destruction of such an individual diode results in a short circuit in that particular diode thereby rendering the entire rectifier inoperative.
Matching of diodes in such rectifiers generally consists of growing the individual diodes from the same crystal and carefully checking to assure that they have similar characteristics. Such a process makes the cost of the complete rectifier substantially more than the sum of the cost of ordinary individual diodes.
In certain rectifier applications such as battery chargers, diode rectifiers must not only be able to withstand normal battery charging currents, but are also called upon to supply currents as much as ten (10) times normal, such as when a motor vehicle to which a battery charger is connected is attempted to be started. An encapsulated diode rectifier of the type previously discussed is easily damaged by excessive current through the rectifier. While the individual diodes making up the rectifier could sustain such excess currents if adequate cooling capability was present, the structure of such an encapsulated rectifier does not permit the rapid dissipation of such excessive heat when such current is applied. In such a rectifier device the heat generated within the rectifier under those conditions tends to destroy the individual diodes. If encapsulated rectifiers are to be used in a battery charger type circuit where such excess currents above normal currents are to be expected, it is necessary to utilize rectifiers having a rated current capacity far in excess of the normal current condition of the circuit. Costs of encapsulated rectifiers increases substantially with increase in current carrying capacity.